The present invention relates generally to an electrical analog of a pumping system and more specifically to an improved crank shaft driving source to be used with a capacitive model of a reciprocating compressor.
Installation or modification of natural gas or other fluid distribution systems requires consideration of a number of factors before work is undertaken. Variations in loads, distribution paths, pipe sizes and compressor speeds all have effects on operation of the system as a whole. Compression waves created in the gas by the operation of reciprocating pumps and compressors are especially troublesome as resonances can be set up in the system. This acoustical resonance increases metal fatigue and shortens the life of joints, valves and other components of the system.
To assist in planning for the control of undesirable pulsation and vibration of compressor and pipeline systems, an electrical analog of all fluid transfer components can be created. Present electrical systems analogize current to mass flow of the gas and voltage to pressure. Inductors, capacitors and resistors are used to model the mechanical properties of pipes and other components in the distribution system. A detailed model of a distribution system or subsystem can be set up and studied to predict the effects caused by changing various parameters in the operation of the distribution system. Examples of the use of gas pumping system analogs are found in U.S. Pat. Nos. 2,951,638 and 2,979,940.
In order to utilize easily obtained components, the operating frequency of the electrical analog is typically substantially higher than that of the mechanical system. An electrical to mechanical frequency ratio describes this relationship. Component values and analog system parameters are chosen so that all events which occur during the operation of the analog reflect events which will take place in the mechanical system. For example, the presence of an electrical resonance in the analog system at a certain frequency corresponds to a mechanical resonance at the corresponding mechanical speed.
One model of a reciprocating compressor or pump includes a capacitor which is driven by an AC voltage source. The signal source models the mechanical input, usually an electric motor or internal combustion engine, used to drive the compressor. Present signal sources used with reciprocating pump analogs are simple variable frequency sinusoidal oscillators.
It is desirable to test the reactions of a proposed or existing compressor or pumping system to various compressor speeds. This is accomplished by changing the driving oscillator frequency in the electrical analog of the pumps and compressors. For a given electrical to mechanical frequency ratio, the electrical frequency of the oscillator can be calculated for a desired mechanical frequency. One drawback of present systems is that a desired change in mechanical speed requires a recalculation of the electrical operating frequency.
A further drawback of present systems is that a change in the electrical to mechanical frequency ratio requires recalculation of all electrical operating frequencies corresponding to desired mechanical frequencies. It would be desirable to slowly vary the operating frequency of the analog compressors in order to study the effect of a range of compressor speeds on the system. Another drawback of present driving signal sources is that this cannot be accomplished.